Middleby Marshall PS360, PS570 Iv. Thermocouples, A. Part Number Reference - Thermocouples

SECTION 3 - SERVICING COMPONENTS

IV. THERMOCOUPLES

All temperature controller and high limit functions are based on input from thermocouples. When two wires composed of dissimilar metals are joined together and one of the ends is heated, a continuous current flow is generated. Middleby ovens use a Type J (iron constant) thermocouple. The iron wire increases the number of dissimilar metal junctions in the circuit.

Middleby has used several types of thermocouples. All can be replaced with the current version. Refer to Figure 24 for illustrations.

•A two-lead thermocouple with a small retaining clip. Used for EITHER temperature-sensing OR high-limit functions. This type is no longer available.

Figure 24 - Thermocouples

	Cable
flange	(3 leads)

Current 3-lead thermocouple

Cable

(2 or 4 leads)

Obsolete 2-lead

or 4-lead

thermocoupleclip

•	A four-lead thermocouple with a small retaining clip.	A. Part Number Reference - Thermocouples
	This is a dual-function unit that sends its signal to the
		Part numbers are provided ONLY for the current, 3-lead ther-
	temperature controller and to a separate high limit con-
		mocouple kits. These thermocouples include instructions
	trol module. This type is no longer available.
		for replacing all previous thermocouple designs, in all oven
•	A three-lead thermocouple with a separate ground lead,
		models.
	a shielded cable and a large mounting flange. This is
			P/N	Description
	the only type of thermocouple available from Middleby
		33984 Kit, thermocouple, PS360-series or PS555/570
	at this time. If it is used to replace a four-lead dual-
	function thermocouple, two complete and separate ther-			series
	mocouples will have to be installed.	33985 Kit, thermocouple, PS200-series or PS536
B. Thermocouple Functions		2.	High limit thermocouple
1.	Temperature-sensing thermocouple(s)		Middleby uses the term “high limit thermocouple” in
	All thermocouples monitor oven temperature. However,		reference to the thermocouple that sends its signal
			solely to the separate high limit control module.
	Middleby uses the term “temperature-sensing thermo-
			If the oven does not have a separate high limit control
	couple” in reference to thermocouples that send their
	signal to the temperature controller instead of to a sepa-		module, there is no high limit thermocouple. On these
	rate high limit control module.		ovens, the temperature-sensing thermocouple(s) con-
	On ovens that do NOT include a separate high-limit		trol high limit activation.
	control module (this includes most PS200-series and	C.	Troubleshooting
	PS360-series gas ovens), this thermocouple signal is
		1.	Open thermocouple
	used to monitor the oven temperature for regulating the

gas flow/heater activation, AND to activate the high limit	An open in a thermocouple will cause the temperature
	controller to read the maximum temperature (usually
warning in case of an overtemp condition.
	909°F/487°C). This causes the following:
Middleby ovens use from one to three temperature-sens-
	• On ovens without a separate high limit control mod-
ing thermocouples, depending on the oven model.
	ule, an open thermocouple triggers the high limit
Temperature-sensing thermocouples are used on all
	alarm. This shuts down the burner. After the open
oven models and are connected to Terminals 7 and 8
	has been corrected, the high limit will need to be
on the temperature controller. Based on the signal from
	reset to allow the oven to resume operation.
this thermocouple, the temperature controller:
	• On ovens with a separate high limit control mod-
• Calls for heat from the burner/heating elements to
	ule, an open temperature-sensing thermocouple
maintain the set point (output from Terminals 4 & 5
	causes the temperature controller to stop calling
for on-off electric and gas ovens; output from Ter-
	for heat. This shuts down the burner.
minals 15 & 16 for variable pulse electric and modu-
	• On ovens with a separate high limit control mod-
lating gas ovens).
	ule, an open high limit thermocouple will trigger the
• Controls the cooldown feature if the HEAT switch
	high limit control module to shut down the burner.
is turned off while the oven is above the cooldown
	Note that the high limit control will need to be reset
temperature (output from Terminals 9 & 10).
	after the open is corrected before the oven can re-
• On ovens without a separate high limit control mod-
	turn to operation.
ule (most PS200-series and PS360-series gas and
electric ovens), the oven temperature sensed by
this thermocouple is also used to activate the high
limit warning (output from Terminals 11 & 12).	54